1. Field of the Invention
The present invention relates generally to a wireless communication system and, more particularly, to a Dedicated Control CHannel (DCCH) transmission method and apparatus for transmitting Dedicated Reference Signal (DRS) in a wireless communication system.
2. Description of the Related Art
Mobile communication systems have been developed in order to provide subscribers with voice communication services while they are on the move. Mobile communication systems have evolved to support high-speed data communication services as well as standard voice communication services. However, limited resource and user requirements for higher speed services in current mobile communication systems have spurred the evolution of more advanced mobile communication systems.
The next generation mobile communication systems of the 3rd Generation Partnership Project (3GPP), Long Term Evolution (LTE) and LTE-Advanced (LTE-A), are currently being standardized. LTE-A is a technology for realizing high-speed packet-based communication at about up to 1 Gigabit per second (Gbps). In order to achieve this speed, several schemes, such as network multiplexing, are being developed for deploying multiple overlapped evolved Node Bs (eNBs) in a specific area and increasing frequency bands supported by an eNB.
In LTE, the control channel is designed based on the distributed transmission concept. This concept aims to minimize and distribute inter-cell interference and maximize frequency diversity gain. LTE-A is designed in consideration of an environment having high inter-cell interference probability due to the very closely deployed cells. Accordingly, the control channel designed based on the distributed transmission scheme inevitably has inter-cell interference. Particularly, in LTE-A supporting Multi-User-Multiple Input Multiple Output (MU-MIMO) transmission, it is difficult to fulfill the performance requirement with the control channel designed for LTE. Accordingly, a new control channel is being developed for transmission on a specific frequency region. Since this specific frequency region is received by a specific User Equipment (UE), the transmission is made based on a dedicated reference signal. A new dedicated control channel region is defined for the UE.
Orthogonal Frequency Division Multiplexing (OFDM) is a transmission technique for transmitting data using multiple carriers. Specifically, OFDM is a multicarrier data transmission technique that parallelizes the serial input stream into parallel data streams and modulates the data streams onto the orthogonal multiple carriers, i.e., sub-carrier channels.
The origin of the multicarrier modulation scheme started in the late 1950's with the microwave radio for military communication purposes. OFDM using orthogonal overlapping multiple subcarriers was developed in the 1970's but was limited in its application to real systems due to the difficulty in implementing orthogonal modulations between multiple carriers. With the introduction of the use of a Discrete Fourier Transform (DFT) for implementation of the generation and reception of OFDM signals, by Weinstein, in 1971, OFDM technology has developed rapidly. Additionally, the introduction of a guard interval at the start of each symbol, and use of a Cyclic Prefix (CP) help to overcome negative effects caused by multipath signals and delay spread.                Owing to such technical advances, OFDM technology is applied in various digital communications fields such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). Specifically, the implementation of OFDM was accomplished by reducing implementation complexity with the introduction of various digital signal processing technologies, such as, Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT).        
OFDM is similar to Frequency Division Multiplexing (FDM), but much more spectrally efficient in achieving high-speed data transmission, by overlapping multiple subcarriers orthogonally. Due to the spectral efficiency and robustness to the multipath fading, OFDM has been considered a prominent solution for broadband data communication systems.
OFDM is advantageous in that it is able to control Inter-symbol Interference (ISI) using the guard interval and reduce the complexity of equalizer in view of hardware as well as spectral efficiency and robustness to the frequency selective fading and multipath fading. OFDM also maintains a robustness against impulse noise so as to be employed in various communication systems.
In wireless communications, high-speed, high-quality data services are generally hindered by channel environments. In wireless communications, the channel environments suffer from frequent changes not only due to Additive White Gaussian Noise (AWGN), but also power variation of received signals, caused by a fading phenomenon, i.e., shadowing, which is a Doppler effect brought about by movement of a UE and a frequent change in a velocity of the UE, and interference by other users or multipath signals. Therefore, in order to support high-speed, high-quality data services in wireless communication, there is a need to efficiently overcome the above channel quality degradation factors.
In OFDM, modulation signals are located in the two-dimensional time-frequency resources. Resources on the time domain are divided into different OFDM symbols, and are orthogonal with each other. Resources on the frequency domain are divided into different tones, and are also orthogonal with each other. Specifically, the OFDM scheme defines one minimum unit resource by designating a particular OFDM symbol on the time domain and a particular tone on the frequency domain. The unit resource is referred to as a Resource Element (RE). Since different REs are orthogonal with each other, signals transmitted on different REs can be received without causing interference to each other.
A physical channel is a channel defined on the physical layer for transmitting modulation symbols obtained by modulating one or more coded bit sequences. In an Orthogonal Frequency Division Multiple Access (OFDMA) system, a plurality of physical channels can be transmitted depending on the usage of the information sequence or the receiver. The transmitter and the receiver determine REs on which a physical channel is transmitted, which is a process referred to as mapping.
The LTE and LTE-A systems are the representative systems, which adopt OFDM in downlink and Single Carrier-Frequency Division Multiple Access (SC-FDMA) in up link.